We received a reply! Dame Vera answered the questions we asked in our letter and signed the photographs we sent to her. A lovely reply from a lovely lady!
Edit: Posted in March, received reply dated 16th of June.

You lucky sods. We wrote to her (same address) about the same time and haven't had any reply at all :-(. We've been excited every time the postman visited, but to no avail... hopefully we'll get a reply soon!